Biomechanics of the Spine
Vertebral Column • Complex structure • Base of support • Link between upper and lower extremities • Transfer load from head trunk to the pelvis • Protects spinal cord • Stability vs. mobility
General Motion Segment • Functional Spinal Unit – 2 adjacent vertebrae & intervening soft tissue – Divided into 2 aspects • Anterior aspect • Posterior aspect
General Motion Segment Transverse Process
Vertebra
Spinous Process
Cortical Shell
Pedicle Facet Joint
Endplate
Disc Cancellous or Trabecular Bone Nucleus Pulposus
Annulus Fibrosus
Anterior Aspect of Spinal Column • Vertebral body • Primary load-transmitting element, 8090% • Increasing size from C to L spine • Compressive load> pressure higher in center of end plates than periphery • In vivo, filled with blood> greater strength, hydraulic shock absorber • Weaker anterior trabeculae, Wolff’s law
Posterior Aspect of Spinal Column • Pedicles, lamina, facet joints, spinous & transverse processes • Bony processes – Lengthen moment arms of muscles
• Forces on processes – Transmitted to Lamina
• Forces on posterior aspects – Transmitted to vertebral bodies from Pedicles
Posterior Aspect of Spinal Column • Pars Interarticularis – Large bending forces; excessive extension – Thicker than rest of lamina
• Facet Joints – Major role in controlling motion – Resist torsion & shear, role in compression – Load sharing varies with flexion & extension – Capsules lax, allow gliding
Facet Joint • Articulation between the superior and inferior facets • Guide intervertebral motion through their orientation in the transverse and frontal planes
Facet Joint Capsule • Limit motions • Strongest in thoracolumbar and cervicothoracic regions where the curvatures change • Resist flexion and undertake tensile loading in the superior portion with axial loading or extension. • Resists rotation in lumbar region
Function of the Facet Joints • Limit the range of motion in the different regions of the spine • To assist in load bearing, sustaining up to 30% of the compressive load on the spine, particularly when the spine is in hyperextension
Facets • Change orientation – From Cervical to Lumbar regions
• Articular joints – Allow smooth articulations between vertebrae
• Resist motion – Through bony interactions – Aided by ligaments
Intervebral Foramina • Exit for nerve root. • The size is dictated by the disc heights and the pedicle shape. • Decreases by 20% with extension and increases 24% with flexion
Spinal Ligaments • • • •
Anterior Longitudinal Posterior Longitudinal Ligamentum Flavum Interspinous Ligaments • Supraspinous Ligaments • Intertransverse Ligaments
Spinal Ligaments • Limit motion, provide stability/equilibrium • Anterior longitudinal ligament – Interlinked to disks – Resists extension – 2X tensile strength of Posterior longitudinal ligament
• Posterior longitudinal ligament – Narrow over vertebral bodies – Resists flexion
Spinal Ligaments • Ligamentum Flavum – Elastic & strong – “shingled” configuration with laminae – Lengthen w/ flexion, shorten w/ extension
• Interspinous & Supraspinous – Resist flexion – Long moment arms
• Intertransverse Ligaments – Resist lateral flexion
Motion Segment Studies • 6 degrees of freedom – Translation & Rotation – 3 orthogonal planes – Motion usually coupled
• Center of gravity – In front of 2nd sacral segment
Intervertebral Disc • Make up 20-30% of the height of the column • thickness varies from – 3mm in cervical region – 5mm in thoracic region – 9 mm in the lumbar region
• Total – 23 discs
• ¼ th of the spinal column's length
Intervertebral Disc • Ratio between the vertebral body height and the disk height will dictate the mobility between the vertebra – Highest ratio in cervical region allows for motion – Lowest ratio in thoracic region limits motion
• Avascular • Nutrients diffuse through end plates
Intervertebral Disc • Spongy center – Nucleus pulposus
• Surrounded by a tougher outer fibrous ring – Anulus fibrosus
Nucleus Pulposus • Is located in the center – Except in lumbar lies slightly posterior
• 80-90% is H2O – decreases with age
• Disc volume will reduce 20% daily (reversible) – Causes a loss of 15-25 mm of height in the spinal column
Annulus Fibrosus • • • • •
Mostly avascular Thickest anteriorly Outermost 1/3 connects to vertebral body Outer 2/3 connect to the end plate Collagen arranged in sheets called lamellae (outer layers). – arranged in concentric rings -10-12 layers that lessen in number with age and thicken
Intervertebral Disc Functions • Movement of fluid within the nucleus – Allows vertebrae to rock back and forth – Flexibility
• Act to pad and maintain the space between the twenty-four movable vertebrae • Act as shock absorbers • Allow extension and flexion
Theory of weight bearing • Develops internal pressure • Pressure exerted in all directions – Lateral forces • Against annulus – Superiorly and inferiorly directed forces • Against end plates – Increases stiffness • Of end plate and annulus fibrosus
Types of Loading • Axial Compression – Caused by gravity, ground reaction forces, muscle contraction and ligaments reaction to tensile forces
Types of Loading • Axial Compression – Causes tension at the annulus, changing the angle of the fibers and increasing the stability – Most load in anterior segment – Posterior segment can load from 0-30% depending upon segments position
Types of Loading • Bending – Combination of compression, shear and tensile forces on the segment from translation
Types of Loading • Torsion – Caused by axial rotation and coupled motions – Annulus fibrosus resists, 1/2 fibers CW other 1/2 CCW in a tensile manner – facets resist depending upon the orientation
Types of Loading • Shear – Annulus will undergo some tensile forces depending upon direction and the fiber orientation or angle
Intradiscal Pressure • Disk pressure is usually uniform • Pressure lowest in supine position • Compressive loads in vivo: 500N standing, 700N sitting • Increased to 3000 to 6000N during lifting of moderate weights, decreases with load closer to body
Mechanical Characteristics
Tensile stiffness of the disc annulus in different directions Highest along – 150 Lowest along – the disc axis
Mechanical Characteristics
Highest – Along normal direction of annulus fibers ( 3 times stronger than that along horizontal direction)
Theory of weight bearing
Spinal Discs under Various Loads
Normal Load
Additional Load
Uneven Load
Torsion Load
Spinal Movement • Spinal movement is the combination of – Intervertebral joints – Facet joints
Back Flexion • Superior vertebra will anterior tilt and forward gliding – Widen the intervertebral foramina 24%. – Add compressive forces on the anterior aspect of the anterior segment – Move the nucleus pulposus posteriorly – Tensile forces will be placed on posterior segment – Central canal is widened
Back Extension • Superior vertebra will tilt and glide posteriorly – The intervertebral foramina narrowed up to 20% – The central canal is narrowed – Nucleus pulposus moves anteriorly
Back Lateral Flexion • Superior vertebra will translate, tilt and rotate over inferior in different direction – Tensile forces on convexity – Compressive forces on concavity – Extension in ipsilateral facet – Flexion in contralateral facet
Rotation • Accessory motions are like lateral flexion due to same coupling in cervical and upper thoracic spine • Exception with lower T/S and L/S in neutral coupling then opposite (in most references)
The Spinal Column • A curved stack of 33 vertebrae structurally divided into five regions: – Cervical region; 7 vertebrae
The Spinal Column • A curved stack of 33 vertebrae structurally divided into five regions: – Thoracic region; 12 vertebrae
The Spinal Column • A curved stack of 33 vertebrae structurally divided into five regions: – Lumbar region; 5 vertebrae
The Spinal Column • A curved stack of 33 vertebrae structurally divided into five regions: – Sacrum; 5 fused vertebrae – Coccyx - 4 fused vertebrae
Spinal Curvatures • Prior to birth “C-shaped” • There are 4 distinct curves in an adult – Primary spinal curves – Secondary spinal curves
Primary Spinal Curves • The thoracic and sacral curves • Concave anteriorly • Are present at birth
Secondary Spinal Curves • The lumbar and cervical curves • Concave posteriorly • Develop from supporting the body in an upright position after young children begin to sit and stand
Secondary Spinal Curves • In the sagittal plane – ‘S’ shape
• As a small child – When starts to sit – Cervical lordosis
• Toddler and adult – When starts to stand – Lumbar lordosis – Allows spring-like action